1. Field of the Invention
The present invention relates to a method and a device for monitoring an exhaust gas turbocharger of an internal combustion engine.
2. Description of the Prior Art
In internal combustion engines, it is known to cause an air/fuel mixture to burn by compressing it. The power output of the internal combustion depends on the ratio of the fuel mass flow rate to the air mass flow rate. The measurement of a respective air mass flow rate is carried out with an air mass flow rate sensor which is seated in the intake section of the internal combustion engine. Numerous modern internal combustion engines are equipped today with an exhaust-gas turbocharger which brings about precompression of the air mass flow. Although attempts at precompression of the air to be fed to an internal combustion engine with the objective of increasing the engine power by increasing the air mass flow rate and fuel flow rate per working stroke were carried out at the very start of the development of the internal combustion engines, nowadays the charging of spark-ignition internal combustion engines is no longer primarily considered from the point of view of power but rather as a possible way of saving fuel and reducing pollutants. In this context, energy for precompressing the air mass flow rate is extracted in a known fashion from a respective flow of exhaust gas by means of a turbine which runs in the flow of exhaust gas, with a fresh air compressor which is mechanically coupled to said turbine, with the result that, a diesel engine no longer operates as an induction engine but rather as a supercharged engine with charge air pressures of up to 1.5 or 2.5 bar with a clear power increase and reduced emissions of pollutants.
Using a turbocharger not only increases the torque of an internal combustion engine but also the thermal loading of the internal combustion engine, for which reason the engine block, cylinder heads, cylinder head seals, bearings, cylinders, connecting rods, valves, pistons and other engine components as well as the subsequent drive train have to be correspondingly configured for this additional loading. The higher power also requires a cooling system for cooling the engine and the charge air to be given correspondingly large dimensions. In this context, it is, however, frequently found in the case of supercharged spark-ignition engines that exhaust gas turbines even become red hot after journeys at a high load. Such strong thermal and mechanical stressing of a component which can reach rotational speeds of up to 200,000 revolutions per minute makes separate monitoring necessary. This is because when turbochargers are used in modern motor vehicles a considerable complexity of control is caused, as a result of which damage diagnosis is made more complicated. Modern completely electronic diagnosis systems have an existing function in this context by evaluating the rotational speed of a turbocharger in order to monitor its function. However, a specially developed sensor has to be provided to determine the rotational speed of a turbocharger. This sensor has to withstand extremely adverse conditions, that is to say, in particular, high temperatures and high pressures, and at the same time reliably detect the blades of the turbocharger wheel and calculate the rotational speed signal with an electronic system connected downstream. For this purpose, the sensor has to be mounted directly on the turbocharger.
Given all the advantages, it is known to be a disadvantage of a turbocharger that a turbocharger cannot generate a sufficient charge pressure for the fresh air in a starting up process/acceleration process as a transient operating state, with the result, that in the intake system an under pressure is briefly produced. When acceleration from low rotational speeds occurs, the correct quantity of exhaust gas to generate the desired charge pressure is initially not available. A sufficiently strong flow of exhaust gas to bring about supercharging to a necessary degree is not available until the rotational speed rises.
This lack of power at low rotational speeds is commonly referred to as a turbo lag. Accordingly, the supercharging of the flow of fresh air through the turbocharger starts with a delay when the throttle is suddenly opened since it is only once there is a sufficient flow of exhaust gas that it can begin. It has been possible in the past to compensate for this property to a certain extent by means of corresponding control systems and by using relatively small turbochargers. Within the scope of a new design approach using the 42 V vehicle on-board electrical system which is currently in planning it would also be possible to provide combined supercharging by means of an exhaust-gas turbocharger and an electric drive. This would additionally change the response behavior in a positive way. However, this approach requires relatively precise knowledge of a current rotational speed of a turbocharger for control in real time. As a result, future control tasks will also only be able to be performed using monitoring of the exhaust-gas turbocharger, which monitoring process supplies a rotational speed of the turbocharger as an output signal.